The reaction of α- and ω-methylenelactams with nitrones. Influence of electronic and geometric factors on the stereoselectivity of their 1,3-dipolar cycloaddition

Various four to six-membered ring α- and ω-methylenelactams 1–5 were reacted with nitrones 6 and afforded good yields of spiroadducts 7–12 through a regiospecific [3 + 2] cycloaddition. Owing to the creation of at least two new asymmetric centres, mixtures of diastereoisomers were usually obtained whose structures were established by two-dimensional NOESY experiments or X-ray crystallography. Among the dipolarophiles, 3-methyleneazetidin-2-ones 1 yielded adducts with a very high stereoselectivity. A similar behaviour was also observed with α-benzoylnitrone 6a as a 1,3-dipole. Electronic and geometric interactions of the reagents 1–5 and 6 in the course of the cycloaddition process were discussed in order to account for the experimental stereochemical outcomes.